Yale law professor Ian Ayres works through United States v. Wegematic Corp., a case about a computer manufacturer that could not deliver a machine it had promised, as the vehicle for teaching the doctrine of impracticability. Drawn from his Coursera course American Contract Law, the lecture asks when a seller's failure to perform should be excused because performance turned out to be far more costly or difficult than expected, and where courts draw the line between ordinary business risk and true impracticability. Ayres walks through the court's reasoning in the Wegematic case, showing how judges weigh foreseeability and allocation of risk between contracting parties. Running thirteen minutes, the lecture is compact but substantive, aimed at students already following the course's sequence on contract performance and excuse doctrines.